Geotechnical properties of deep sea sediments: Bermuda rise

Abstract

The Giant Piston Corer has been used to obtain good quality sediment samples from several sedimentological/acoustic regimes in the northwestern Atlantic. A total of 185 m of sediment were recovered in nine cores with lengths ranging from 6.5 to 30.5 m. Sediment types include calcareous ooze, pelagic brown clay, organic‐rich gray‐green silty clay, and graded sand. Data are presented from three of these cores, taken from three different acoustic provinces. Seismic profiles and 3.5 kHz records from our study area on the eastern Bermuda Rise (4–5 km water depth) reveal marked differences in acoustic characteristics that are consistent with physical property variations at three of the core sites. This area contains three major features: (l)a broad, gently rolling plateau covered with about 300 m of sediment containing numerous conformable reflectors; (2) a slope region with maximum slope angle of 0.275 rad (16°), which is underlaid by acoustically transparent sediments; and (3) a region at the toe of the slope consisting of sediment hummocks with irregular hyperbolae and occasional conformable reflectors. The illitic sediments of the plateau region are a mixture of glacial and pelagic material with high rates of accumulation (20 to over 200 cm/1000 yr). The water content profile reveals large vertical variability and several zones of distinct contrasts. Shear strengths are low (average of 0.040 kg/cm²) within the upper 15 m where there is no significant increase in strength. Except for the upper 2 or 3 m, which is overconsolidated, test results indicate a significant amount of underconsolidation for most of the 29 m core length. Three correlations have been made between acoustic reflectors and physical properties of the plateau core. All of these reflectors occur at zones of high water content in the core. The pelagic sediments of the slope region contain significant montmorillonite and rates of accumulation are about 1–3 mm/1000 yr. There are few fluctuations in water content within the cored depth of 14 m, the shear strength shows a steady increase with depth, and the sediment is essentially normally”; consolidated. Slope stability analyses, assuming the sediment properties of the plateau region, indicate that slumping due to gravity alone would occur after the accumulation of only a few meters of sediment on this slope. The acoustically transparent seismic record correlates with the lack of significant fluctuations in geotechnical properties. The lithological and water content variability of the hummocks at the base of the slope is similar to that on the plateau region. The sediment column appears to be normally consolidated, and the shear strengths here are considerably higher than those on the plateau. It is concluded that the hummocks are slumped materials that have been transported downslope intermittently.